Graphene is a single- or few-layer structure consisting of sheets of sp2 hybridized carbon atoms. This material has been the subject of considerable research activity in recent years due to its useful mechanical and electrical properties. A ready source of graphene is bulk graphite, which consists of a large number of graphene sheets held together through van der Waals forces. Single- and few-layer graphene sheets have been prepared in microscopic quantities by mechanical exfoliation of bulk graphite (commonly referred to as the “Scotch-tape” method) and by epitaxial chemical vapor deposition. However, these routes are not suitable for large-scale manufacturing of graphene.
To date, methods for preparing bulk quantities of graphene have centered on chemical exfoliation of graphite. The most common approach for exfoliation of graphite has been to use a strong oxidizing agent to produce graphene oxide, a non-conductive and hydrophilic carbon material. Although the exact chemical structure of graphene oxide is difficult to conclusively determine, it is at least qualitatively evident that the regular sp2 structure is disrupted in graphene oxide with epoxides, alcohols, carbonyls and carboxylic acid groups. The disruption of the lattice in bulk graphite is reflected in an increase in interlayer spacing from 0.335 nm in bulk graphite to more than 0.625 nm in graphene oxide. Graphene oxide was first prepared in 1859 through adding potassium chlorate to a slurry of graphite in fuming nitric acid. The synthesis was improved in 1898 by including sulfuric acid in the reaction mixture and adding the potassium chlorate portionwise over the course of the reaction. The most common method used today is that reported by Hummers in which bulk graphite is oxidized by treatment with KMnO4 and NaNO3 in concentrated H2SO4 (Hummers' method). It should be noted that all three of these procedures involve the generation of the toxic and/or explosive gas(es): NO2, N2O4, and/or ClO2.
Non-conducting graphene oxide may be transformed back into a conductive graphene material, either in thin films or in bulk, through chemical reduction to form chemically converted graphene. However, chemical reduction does not fully restore the pristine sp2 structure of bulk graphite, and significant defects in the form of holes are present in the chemically converted graphene structure. These defects arise during chemical exfoliation of bulk graphite and are not repaired during the reduction of graphene oxide into chemically converted graphene. The defects in both graphene oxide and chemically converted graphene diminish the desirable mechanical and electrical properties of these materials compared to pristine graphene.
In view of the foregoing, chemical methods for exfoliating bulk graphite to produce highly oxidized graphene oxide having a more regular sp2 structure would be of significant benefit in the art. Graphene oxide having a more regular sp2 structure would be capable of being reduced to chemically converted graphene having properties more commensurate with pristine graphene sheets. Further, methods for production of graphene oxide that avoid generation of toxic byproducts would also confer significant advantages to such methods.